1. Field of the Invention
The present invention relates generally to a system and method for predicting acoustic loads that may occur during operation of a boiling water reactor (BWR), and more particularly, to develop a pressure load definition using an analytical acoustic model that may be used as input for an analytical structural model of a BWR steam dryer to predict stresses in the BWR steam dryer.
2. Description of the Related Art
A reactor pressure vessel (RPV) of a nuclear reactor such as a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the reactor core and is supported by a shroud support structure. The shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud.
Heat is generated within the core and water circulated up through the core is at least partially converted to steam. Steam separators separate the steam and the water. Residual water is removed from the steam by steam dryers located above the core. The de-watered steam exits the RPV through a steam outlet near the vessel top head.
Conventional BWRs can experience damage resulting from aero-acoustic loading of the steam dryer during operation. Some conventional BWRs have experienced significant degradation and/or failures of the steam dryer after operating at power levels in excess of the original licensed thermal power.
For example, steam dryer failures may occur due to high cycle fatigue caused by pressure oscillations generated when the vortex shedding frequency associated with flow over the steam dryer and other discontinuities, for example, Safety Relief Valves (SRVs) coincide with certain acoustic natural frequencies of the steam system.
Steam dryer damage can prevent the plant from operating at a desired power level. Further, costs (time, money, etc.) associated with repairs to the steam dryer can be significant. Accordingly, it is desirable to be able to predict the nature of acoustic loads expected on a BWR steam dryer at various power levels so that a structural evaluation of a full-size BWR steam dryer may be performed prior to operating at a higher power level.
Conventionally, there are several methods used to predict the nature of the acoustic loads expected on BWR steam dryers. These methods include (1) empirical generic load estimates based on in plant operating data from different BWR configurations and different operating conditions; (2) plant specific in-vessel instrumentation programs to measure acoustic loads at various power levels; (3) acoustic circuit models of a plant configuration driven by in-plant data obtained at desired power level from instrument lines or main steam line strain gauges; and (4) Computational Fluid Dynamics (CFD) analyses performed for a plant specific configuration.
The empirical generic load estimate is inaccurate and hampered by the fact that the data is obtained from reactor plants other than the plant considered. Thus, no plant-specific information is used to determine if the load estimate is conservative or non-conservative for the plant being considered. This method uses all information available from a BWR steam system in an attempt to produce an acoustic load definition for any plant. The suitability of this method for plant specific applications is difficult to demonstrate. Many utilities complain that the load prediction is too conservative. The Nuclear Regulatory Commission (NRC) complains that the empirical method is not sufficient to differentiate between plants that have experienced steam dryer failures and plants which have not.
In some cases, utilities have decided to pursue in-vessel instrumentation programs to measure actual loads on the steam dryer. However, this method is expensive, which makes it an undesirable approach for many utilities. Further, this method is channel limited, meaning that a limited number of instruments may be placed on the steam dryer to obtain operating data. This number is typically around 40 instrument locations. The limited number of instrument locations inhibits and/or prevents the creation of a fine mesh load definition used in a Finite Element Analysis (FEA). Use of in-vessel instrumentation also requires that the critical regions of the steam dryer be known prior to the time that the in-vessel tests are performed. Further, there is no opportunity to relocate instruments once the reactor is back online and operational. Still further, this method is not predictive. Loads can only be calculated after data is obtained from the plant at the operating conditions that the acoustic loads are desired. It is not possible to extrapolate the expected loads at high power levels using measurements at lower power levels due to the non-linear behavior of the acoustic resonances that cause steam dryer failures. Accordingly, a plant would actually need to operate at potentially damaging power levels in order to obtain relevant data.
Further, some organizations have created acoustic circuit approximations of a plant specific steam system. These analytical models are effectively transfer functions used to predict acoustic loads on the steam dryer from unsteady pressure data obtained from instrumentation lines attached to the RPV, main steam lines or main steam line strain gauges. The acoustic circuit models and methods cannot be used to predict plant-specific loads unless data is obtained from the plant at the operating conditions of the desired acoustic load conditions. The unsteady pressure data is obtained at the end of instrumentation lines containing both liquid water and steam, and thus exhibits significant thermal gradients. The condition of the instrument lines makes an accurate prediction of the unsteady pressure in the steam lines difficult to verify. Additionally, use of main steam line strain gauges provides data that contains mechanical signals introduced into the desired acoustic pressures by main steam line vibration; thus a large number of strain gauges and significant signal processing care must be taken to apply this method. In other words, prediction of the system response in one portion of the system using the response from another portion of the system, without a complete understanding of the location and characteristics of all acoustic sources, makes it difficult to verify the load predictions obtained with this method.
Some CFD analyses have been performed in an effort to understand the loading expected on the steam dryer. However, the lack of empirical data to benchmark this approach, the physical size of the model required to approximate the steam system, and the computational resources required to make an accurate prediction of unsteady pressure oscillations on the steam dryer prevent this approach from being practical. This technology is not yet mature enough to be used for an industrial problem of the complexity exhibited by the BWR steam system.